- Neurotransmitters are chemical agents synthesized by nerve cells.
- Are stored in secretory vesicles in axon terminals.
- Released when a nerve impulse reaches the end bulb.
- Also defined as the chemicals which allow the transmission of signals from one neuron to the next across synapses.
- Produce an excitatory or inhibitory response in the post synaptic membrane.
- Released from synaptic vesicles in synapses into the synaptic cleft.
- In synaptic cleft, they are received by neurotransmitter receptors on the target cells.
- For example: acetylcholine evokes an excitatory response at the motor end plate in a skeletal muscle cell. whereas it evokes an inhibitory response at its synapse with a cardiac muscle cell.
- Many possible neurotransmitters are being investigated.
- There are four main criteria for identifying neurotransmitters:
- The chemical must be synthesized in the neuron or otherwise be present in it.
- When the neuron is active, the chemical must be released and produce a response in some target.
- The same response must be obtained when the chemical is experimentally placed on the target.
- A mechanism must exist for removing the chemical from its site of activation after its work is done.
- About 50 are present in our nervous system and there may be over 100.
- Known neurotransmitters are classified into four groups. They are:
- It is the main neurotransmitter released by neurons of the peripheral nervous system.
- Only a restricted number of neurons in the central nervous system release acetylcholine.
- The acetylcholine released by lower motor neurons stimulates contraction of skeletal muscle.
- In contrast, the acetylcholine released by the postganglionic neurons of the peripheral nervous system inhibits contraction of cardiac muscle.
2) The amino acids neurotransmitters
- Amino acid neurotransmitters release (exocytosis) is dependent upon calcium Ca2+and is a presynaptic response.
- There are inhibitory amino acids (IAA) or excitatory amino acids (EAA).
- Some of them include gamma-aminobutyric acid (GABA), glycine, aspartic acid, and glutamic acid.
- Although GABA is not an amino acid, it is classified with the amino acids because it is derived from glutamic acid.
- It is the major inhibitory transmitter of the small local circuit neurons in such structures as the cerebral cortex, cerebellum, and upper brainstem.
- Glycine, the simplest amino acid, is the major inhibitory transmitter of local circuit neurons in the lower brain stem and spinal cord.
- It selectively inhibits certain neurons to refine and focus brain activity into meaningful patterns by eliminating and suppressing nonessential activity.
- Glutamic acid and aspartic acid are excitatory transmitters in the central nervous system.
- Monoamine neurotransmitters are neurotransmitters and neuromodulators.
- Contain one amino group connected to an aromatic ring.
- connection occurs by a two-carbon chain (such as -CH2-CH2-).
- Include catecholamines, such as norepinephrine and dopamine, and indoleamines, such as serotonin.
- Nor-ephinephrine (noradrenaline) is located in neurons with cell bodies in the brain stem and in post-ganglionic neurons of the sympathetic nervous system.
- Norepinephrine releasing fibers are distributed widely in structures such as the cerebral cortex, cerebellum and spinal cord.
- Fibers that terminate in the cerebral cortex are involved with various levels of consciousness.
- Pep pills containing the drug amphetamine increase the level of norepinephrine in the brain by blocking its reuptake in the neuron and inhibiting the action of enzyme monoamine oxidase (MAO).
- The euphoria and hallucinations produced by amphetamines are thought to result from the inhibition of MAO.
- An amphetamine psychosis which is quite similar to schizophrenia can result from the habitual use of amphetamines.
- Chlorpromazine is used in the treatment of schizophrenia and similar disturbances.
- It acts by blocking the reuptake of norepinephrine.
- Dopamine is neurotransmitter found in the area of brain where certain neural circuits are involved with voluntary motor integration.
- Deficits of dopamine involving the basal ganglia are associated with Parkinson’s disease.
- Serotonin is associated with various mood swings, including depression, elation, insomnia, and hallucinations.
- Chains of neuroactive aminoacids.
- Found in the central nervous system (CNS).
- Somatostatin, the endorphins and the enkephalins are examples.
- Release of growth hormone from the pituitary gland is acted upon by somatostatin.
- Inhibitory function is shown by somatostatin to growth hormone.
- Apparently, it acts by altering the excitability of the post synaptic membrane of target cells.
- Endorphins (morphine like substances) and enkephalins (in the head substances) are naturally occurring peptides.
- Found in several regions of the central nervous system.
- Function at post synaptic receptor sites in the pain pathways.
- Suppress synaptic activity leading to pain sensation.
- These opiates are not used therapeutically because their effectiveness is relatively short-lived and because they are addictive.
- Many neuropeptides are classified as neuromodulators.
- They are the chemical agents that are capable of altering the responsiveness of neurons to a neurotransmitter.
- Neuropeptides, histamines, prostaglandins, and the hormones cortisol and estrogen are thought to have neuromodulator functions.
Various neurotransmitters and their functions